Are mammals symmetrical? I think most mammals will have asymmetry, like heart on one side, liver on another (in fact, there’s a medical condition that is when a person has all their organs on the opposite side to normal).
If you’re talking more general symmetry, like two eyes, nose in the centre, two arms, etc, (i.e. if you put a mirror in the middle it would still look like the same creature), I can’t come up with any good counter examples. I think some creatures, like owls, have asymmetrical ear positions so they can tell direction better, if that counts? I’m afraid you probably need a zoologist for this question! 🙁